1. Field of the Invention
The present invention relates to an acid anhydride-containing one package epoxy resin composition having excellent processability and storage stability, which is useful for impregnating and casting and may rapidly harden under heat to provide a cured product having excellent electrical, thermal, and chemical properties.
2. Discussion of the Background
A low viscosity epoxy resin composition containing an acid anhydride as a curing agent is useful for infiltrating narrow slits. Because the resulting cured product has excellent electrical, mechanical, thermal, and chemical properties, it is widely used in various fields. For example, it is used as a sealant for condensers, an impregnant for coils, and a sealant for transformers of large-size motors, generators, and magnetic heads. In addition, the uncurred composition possesses excellent storage stability and a long pot life.
However, a composition comprising an epoxy resin and an acid anhydride requires a high temperature and a long time for curing. In order to overcome this drawback, tertiary amines or their salts (JP-A 64-60625, 59-140220-- the term "JP-A" as used herein means an "unexamined published Japanese patent application"), imidazoles or their salts (JP-A 58-83024, 57-19001), organic phosphines (JP-A 63-304018, 57-40524), quaternary phosphonium salts (JP-A 1-25487, 61-151230) or the like are used as curing accelerators. Incorporating such curing accelerators into the composition improves hardenability but decreases storage stability. As a result of the poor storage stability of compositions containing such curing accelerators, it is necessary to prepare the compositions immediately prior to use. Thus, the epoxy resin, acid anhydride, and curing accelerator must be metered separately. Such an operation decreases processability and workability of the composition and causes metering errors. In addition, the excess stock of the composition cannot be stored but must be discarded, causing waste which is environmentally disadvantageous.
The use of a salt of p-toluenesulfonic acid and a tertiary amine such as triethylamine as a curing accelerator for the composition has been investigated (JP-A 60-4520). Although the storage stability of the resulting composition is good, the reactivity is not good.
In order to overcome both problems of storage stability and reactivity, the use of a dispersible latent curing accelerator has been investigated (JP-A 60-4524, 60-72917). A resin composition containing such a dispersible latent curing accelerator almost satisfies the storage stability and reactivity requirements. However, since the curing accelerator is an insoluble powder, the composition containing the insoluble powder cannot penetrate into narrow slits. As a result, the physical properties of the hardened product are unacceptable.
Thus, it is desirable to produce an acid anhydride-containing one package epoxy resin composition having excellent storage stability and reactivity while maintaining excellent processability and workability.